Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

A new study found that a low-expression form of the gene monoamine oxidase A (MAOA) seems to give women good feelings, but it doesn't have the same effect in guys. In fact, it's the same gene type that has been linked to aggression in men.

"This is the first happiness gene for women," lead author Henian Chen, of the University of South Florida, said in a statement.

MAOA controls the activity of an enzyme that breaks down serotonin, dopamine and other feel-good brain chemicals. The low-expression version MAOA leads to higher levels of monoamine, which, in turn, allows larger amounts of these neurotransmitters to stay in the brain and boost mood.

In the study, the researchers examined how this gene was expressed in 193 women and 152 men. They also analyzed the subjects' self-reported happiness.

The researchers found that women with even just one copy of the low-activity type of MAOA were much happier than women with no copies. Meanwhile, many of the men in the study had a copy of the gene variant, but reported no more happiness than their peers without it, the researchers said.

The study, which appears in the journal Progress in Neuro-Psychopharmacology & Biological Psychiatry, controlled for various factors, including age, education and income.

"I was surprised by the result, because low expression of MAOA has been related to some negative outcomes like alcoholism, aggressiveness and antisocial behavior," Chen said in a statement. "It's even called the warrior gene by some scientists, but, at least for women, our study points to a brighter side of this gene."

Chen and his team speculated that testosterone might be to blame for this genetic gender gap, saying the hormone might mitigate the positive effects of MAOA in men.